Magnetic resonance imaging (MRI) apparatus typically employ three gradient coils to produce spatially selective information encoding in a sample. The three gradient coils contain numerous turns of conductive wires and produce gradients that may be pulsed on and off. MRI apparatus also use radio frequency (RF) transmit coils to produce spin excitation in a sample. An RF shield may be placed between a set of RF transmit coils and the gradient coils to prevent the RF field produced by the RF transmit coils from interacting with the gradient coils.
An ideal RF shield would be completely transparent to the gradient field produced by the gradient coils and completely opaque to the RF field produced by the RF transmit coils. Being completely opaque (e.g., completely blocking the RF field from interacting with the gradient coils) is a design goal because an interaction between the RF coils and the gradient coils could produce an RF energy loss that would appear as a lowering of the quality factor Q of the RF coil and would decrease the signal-to-noise ratio (SNR) of signal received from the sample.
Two types of eddy currents may be produced in an RF shield interposed between gradient coils and RF transmit coils. The gradient coils may induce eddy currents in the RF shield and the RF transmission coils may also induce eddy currents in the RF shield. The eddy currents have different properties. RF shields are designed to disrupt the gradient coil induced eddy currents and to not disrupt the RF transmission coil induced eddy currents.
Conventional RF shields are made of a copper-dielectric-copper laminate structure with slits in both copper layers. The slits on the copper layers of the RF shield are designed to suppress gradient eddy current heating while still allowing the RF shield to reduce coupling between the RF and gradient systems. The slits need to be placed to reduce gradient eddy currents while still allowing a return path for RF eddy currents. Therefore, capacitors may be positioned to span a slit in the copper so that RF eddy currents have a return path and are not disrupted. The capacitors form a high-pass filter that allows the RF eddy currents to flow and thus shield the RF coil field. The capacitors also impede gradient eddy currents that might negatively impact the gradient field. Unfortunately, having too many slits and too many capacitors may produce undesirable results. The undesirable results may be produced in traditional RF shields that have hundreds of capacitors placed around multiple small cross-cuts with both axial and azimuthal directions.
The undesirable results include, for example, capacitors being impacted by ohmic heating. Ohmic heating may affect capacitor lifetimes, capacitor failure rates, and melting of soldered bases. As temperatures increase in hybrid systems using split coils, ohmic heating issues with capacitors may also increase. Therefore reducing the numbers of slits and thus the number of capacitors may produce improved results. Additionally, cutting slits in the copper sheets introduces holes through which RF energy can pass. Having RF energy pass through the RF shield makes the RF shield less opaque to the RF field and allows conditions where there could be an interaction between the RF system and the gradient system.
One conventional RF shield includes slits that follow RF current streamlines on the RF shield. While this conventional shield may be appropriate for some applications, it may not be appropriate for circular or “quadrature” excitation where the RF current streamline rotates with the Larmor frequency over the RF shield. As the current streamline rotates with the Larmor frequency it may pass over slits that are fixed in space and thereby allow RF leakage.
Eddy currents that are induced in an RF shield due to the pulsing gradient field can reduce penetration of the gradient field into the imaging volume and may give rise to ohmic heating in the RF shield. This heating in the RF shield may be more pronounced for some rapid imaging techniques (e.g. echo planar imaging (EPI)) and for hybrid MRI systems configured with split gradient coil designs.
MRI-guided hybrid systems are becoming more important due to advantages they provide over traditional MRI systems. In some MRI-guided hybrid systems, the MRI scanner is split into two halves in order to accommodate complementary diagnostic and therapeutic equipment for performing radiotherapy, positron emission tomography (PET), surgery (e.g., ablation), and other applications. In addition to the gap in the MRI main magnet, the X, Y and Z gradient coils may be split.
Gradient coil patterns for hybrid systems may be bunched more closely near gaps in the split coils. FIGS. 1 and 2 illustrate the bunching of gradient coil wires near the gap forming the split between portions of the gradient coil. The closer bunching of gradient coil patterns may exacerbate heating because of stronger local magnetic fields near the denser collection of gradient coil wires. FIG. 1 illustrates a split transverse gradient coil 100. Coil 100 is separated from RF transmit coils 110 by an RF shield 120. Note the different density of coil wires in regions 112 and 114 of coil 110. FIG. 2 illustrates a split longitudinal gradient coil 200. Coil 200 is separated from RF transmit coils 210 by an RF shield 220. Note the different density of coil wires in regions 212 and 214 of coil 210. The higher density of coil wires in regions 114 (FIG. 1) and 214 (FIG. 2) produce stronger local fields that can increase ohmic heating in specific regions on an RF shield. The ohmic heating is produced by oscillating eddy currents produced by rapid pulses in the gradient coils. The concentration of coil wires in regions 114 (FIG. 1) and 214 (FIG. 2) may concentrate ohmic heating. Thus, the issue of ohmic heating may be exacerbated on RF shields used in split geometries as compared to non-split geometries.